1. Field of the Invention
The present invention relates to blends of cationic starch and cationic galactomannan gum such as guar, locust bean, fenugreek gum, etc. useful as strength and drainage aids in papermaking.
2. Description of the Prior Art
Cationic starch and cationic guar gum are used effectively as strength and drainage aids by the paper industry. Typically, only one of these products is added to the wet end of the paper machine. Cationic starch is more commonly used. However, there has been reports of separate addition of these products at different points to a paper machine. Cationic starch is commonly added earlier in the process to provide strength. Cationic guar gum is added closer to the paper machine to provide drainage and fiber retention.
Up to 40 pounds of cationic starch can generally be added to obtain desired strength benefits. Cationic guar gum addition levels are usually in the 1- to 8-lb. range. Both products are supplied to paper mills in powder form. Cationic starch is typically slurried in water at 10-15% solids, subjected to high temperatures, and diluted before addition to the paper machine. High levels of starch can create problems in the process such as increased biological oxygen demand (BOD) and starch build-up, lowering starch etfectiveness. Cationic guar gum must be dispersed using an eductor, and dissolved in water at low solids, typically less than 1%. High amounts of fresh water needed for making guar gum solutions are becoming more difficult to get, as paper mills are closing their water systems. Dust associated with guar-based powder are also safety hazards, which can lead to falls and respiratory problems. Cost and handling problems associated with cationic guar gum have limited its use in paper dry strength applications.
Larsson, U.S. Pat. No. 4,755,259 discloses the separate addition to the papermaking machine of cationic starch, cationic guar and colloidal silica. Larsson does not disclose the addition of a combined cationic starch cationic guar blend in the papermaking machine.
Rushmere, U.S. Pat. No. 4,927,498 discloses the separate addition to the paper machine of cationic polymer, such as either cationic starch or cationic guar gum or cationic polyacrylamide, and anionic polysilicate microgels. The cationic polymers are used interchangeably and not added together as a blend.
"Application of New Gums for Enhancing Strength and Productivity", K. E. Burnfield, TAPPI Press, Apr. 26, 1995, discloses the use of cationic guar gum added to a papermaking system which contains cationic starch in order to improve drainage of the starch containing system. The cationic guar gum and the cationic starch were added separately to the papermaking system and not as a blend.
Before the discovery of the present invention it was considered by those familiar with starch and guar gum chemistry to be difficult or nearly impossible to blend dry cationic starch and dry cationic guar gum for the purpose of adding them to the papermaking machine using conventional techniques, i.e., starch cookers.